1. Field of the Invention
The present invention relates to a control apparatus for a continuously variable transmission provided to attain a fast response ability of speed ratios, particularly to a shift control apparatus having a superior shift control feeling for effectively decreasing a fuel consumption by reducing a load of a pump and by quickly shifting to an optimum speed ratio.
2. Description of the Related Art
A belt-type continuously variable transmission (CVT) which is applied to a transmission of a vehicle has a primary pulley provided on a primary shaft which has a pulley groove of which width is variable and a secondary pulley provided on a secondary shaft which has the pulley groove of which width is variable comprising a belt round between the primary pulley and the secondary pulley. Changing pulley ratios or speed ratios on the primary pulley can continuously change a speed of the secondary shaft.
A primary cylinder with a primary oil chamber is provided in the primary pulley, and the groove width of the primary pulley changes with a volume of a hydraulic oil in the primary oil chamber, whereby the speed ratio is controlled. On the other hand, a secondary cylinder with a secondary oil chamber is provided in the secondary pulley, and a pushing force is transmitted is to the belt wound on the secondary pulley by controlling the volume of the hydraulic oil in the secondary oil chamber.
A shift control method for the continuously variable transmission has two types. A first method is to control the volume of the hydraulic oil in the primary oil chamber and the secondary oil chamber by oil pressure control valves, respectively, and a pressure of the hydraulic oil supplied to the primary oil chamber is directly controlled, whereby the speed ratios of the transmission are controlled. Then, a second method is to control the volume of the hydraulic oil is supplied in the primary oil chamber via a flow rate control valve, so that the amount of the hydraulic oil that flows in and out of the primary oil chamber is regulated, whereby the speed ratio of the transmission is controlled (as disclosed in the related art: JP-A-9-210189).
In the first method, the oil pressure supplied in the primary oil chamber is regulated by the oil pressure control valve for a shift control, and the oil pressure in the primary oil chamber is fed back to a pilot port of the oil pressure control valve. Therefore, when a torque is unexpectedly inputted to the transmission from the wheels due to a change of road surface conditions, for example, when the vehicle runs on a rough road or a road with other friction coefficients, the oil pressure control valve is actuated by an oil pressure feedback function from the pilot port, and the hydraulic oil is caused to flow out or in the primary oil chamber. Accordingly, the speed ratio become unstable. To solve this problem, an orifice or a value is provided in a hydraulic circuit. However, providing the value deteriorates the shift response at a normal time and hence lowers an upper limit of a shifting speed.
On the other hand, it becomes difficult to control the speed ratio, since with the first method it is not possible to estimate the speed ratio by measuring the amount of a hydraulic oil within the cylinder, while it is in generally impossible to detect the rotating speed for calculating the speed ratio when the rotating speed are low,in the control system in which the speed ratio is obtained through arithmetic operations by using rotating speed ratios between the two pulleys, i.e., the primary and secondary pulleys, in order to control the speed ratio.
In the second method, the amount of the hydraulic oil that flows in or out of the primary oil chamber is controlled by the flow rate control valve, and since the flow rate control valve cannot control a pressure, in particular, when attempting a quick shift, there occurs a possibility that the clamping force becomes too large or too small. This affects a durability of the belt and increases the possibility of a belt slippage. Then, in order to solve the problem, when the flow rate control valve is set on a safer side or the spool opening is narrowed to prevent the clamping force from becoming too large or too small when the shifting speed is lowered, resulting in deterioration in a shifting response ability.
In addition, since the flow rate control valve does not control the pressure, the phenomenon cannot be avoided in which a high pressure is generated in the hydraulic circuit for shifting to supply the hydraulic oil to the primary cylinder. Thus, it is needed to avoid this phenomenon, which is disadvantageous in costs. Furthermore, in case the flow rate control valve fails in a released condition, the speed ratio is fixed to the overdrive side, resulting in an insufficient drive force when the vehicle starts from a standstill.
On the other hand, as has been described above, in the control system in which the speed ratio is calculated from the rotating speed ratio between the two pulleys, i.e., the primary and secondary pulleys, in order to control the speed ratio, while it is generally impossible to detect the rotating speed for calculating the speed ratios when the rotating speed is low. In the second method, since the pressure within the cylinder cannot be controlled, it becomes difficult to quickly return the speed ratio to a low-speed side while preventing the occurrence of the belt slippage.
An object of the present invention is to provide a shift control apparatus for a continuously variable transmission which has a superior shift feeling.
Another object of the present invention is to provide the shift control apparatus for the continuously variable transmission which can reduce a fuel consumption by reducing a load of a pump.
A further object of the present invention is to provide the shift control apparatus for the continuously variable transmission which can reduce the fuel consumption by quickly shifting to an optimum speed ratio.
According to an aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission having a primary pulley in a primary cylinder and a secondary pulley in a secondary cylinder, whereby a continuously variable shifting is achieved by changing a diameter of the pulleys by supplying a primary pressure into the primary cylinder, including: a target primary pressure setting unit provided a target primary pressure according to a speed ratio deviation between an actual speed ratio between the primary pulley and the secondary pulley and a target speed ratio according to running conditions of a vehicle; a shift control valve for regulating a flow rate of the hydraulic oil supplied to the primary cylinder by changing an opening area of the shift control valve according to an electric current supplied to an electromagnetic coil; a primary pressure detecting unit for detecting the primary pressure supplied to the primary cylinder; and a control unit for controlling the speed ratio by correcting a basic current based upon a target primary pressure and supplied to the electromagnetic coil by feeding back the primary pressure detected by the primary pressure detecting unit.
According to another aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the target primary pressure is set by an oil pressure ratio and a speed ratio deviation, the oil pressure ratio being between a primary pressure in a steady state corresponding to a target speed ratio and a secondary pressure supplied to the secondary cylinder.
According to a further aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the target primary pressure is set by adding a compensation value calculated by a parameter representing an operating condition such as the speed ratio deviation.
According to the other aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the feedback correction of the primary pressure detected by the primary pressure detecting unit is unexecuted when the variation rate of the target primary pressure is smaller than a predetermined value and when the pressure deviation between the target primary pressure and an actual primary pressure is equal to or smaller than a predetermined value.
According to the other aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the feedback correction of the primary pressure detected by the primary pressure detecting unit is unexecuted when the variation rate of the target speed ratio is smaller than a predetermined value and when the speed ratio deviation between the target speed ratio and the actual speed ratio is equal to or smaller than the predetermined value.
According to the other aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein a current resulted from adding a feedback compensation current obtained by the pressure deviation between the target primary pressure and the actual primary pressure is supplied to the shift control valve.
According to the other aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the shift control valve is closed when the variation rate of the target primary pressure is equal to or smaller than the predetermined value, whereas the opening area of the shift control valve is regulated according to the variation rate when the variation rate is equal to or larger than the predetermined value.
According to the other aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the opening area is regulated by limiting the current supplied to the electromagnetic coil so as to avoid an excessive shifting speed.
According to the other aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the current is limited according to the primary pressure.
According to the other aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein an oil pressure limiting unit for limiting the primary pressure supplied to the primary cylinder is provided in a primary oil passage connected to the primary cylinder, the oil pressure limiting unit inhibits that the primary pressure supplied to the primary cylinder exceeds the predetermined value.
According to another aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the oil pressure limiting unit is a relief valve.
According to the other aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the oil pressure limiting unit is controlled by an output signal from the primary pressure detecting unit.
According to the other aspect of the present invention, there is provided the shift control apparatus for the continuously variable transmission, wherein the amount of the hydraulic oil flown in and out of the primary cylinder is estimated as an estimated valve by the opening area obtained from a relationship between the opening area and the current and the primary pressure when a vehicle is running at a low speed, and wherein the speed ratio is controlled based on the estimated value.
According to the other aspect of the present invention, there is provided a shift control method for a continuously variable transmission having a primary pulley with a primary cylinder and a secondary pulley with a secondary cylinder, whereby the continuously variable shifting is achieved by changing a winding diameter of a belt the primary and secondary pulleys, the method having steps of setting a target primary pressure according to a speed ratio deviation between an actual speed ratio between the primary pulley and the secondary pulley and a target speed ratio calculated according to running conditions of a vehicle, regulating a flow rate of a hydraulic oil supplied to the primary cylinder by changing an opening area of the shift control valve according to a current supplied to an electromagnetic coil, detecting the primary pressure supplied to the primary cylinder, and controlling the speed ratio by correcting a basic current based upon the target primary pressure and supplied to the electromagnetic coil by feeding back the primary pressure detected by the primary pressure detecting unit.
In the present invention, as the shift control valve, since the flow rate control valve in which the flow rate of the hydraulic oil to the primary cylinder is regulated by changing the opening area according to the current supplied to the electromagnetic coil is used, a speed ratio stability is ensured against an unexpected torque input change, and a quick change in the speed ratio can be achieved by obtaining a final target primary pressure from the target speed ratio. The occurrence of a belt slippage can be prevented by monitoring the primary pressure at any times by the primary pressure detecting unit, whereby the shift control can be realized for preventing the occurrence of the belt slippage, which is stable in shift and which can improve the shift response and a shift feeling.
The secondary pressure does not have to be set slightly high for safety when attempting to prevent the occurrence of the belt slippage at the time of shifting, and the oil pressure can be reduced, which then allows the load of the pump to be reduced, thereby making it possible to improve not only fuel consumption but also the durability of the belt.
Since a proper upper limit for the primary pressure can be maintained by the relief valve, the maximum pressure for the primary circuit at a downstream of the relief valve can be set low, and therefore, the transmission can be made light in weight and low in cost.